Improve cross-links between benchmark contract and artifact schemas

[Permea-lab-admin]

Goal

Help readers navigate between benchmark definitions and expected artifacts.

Suggested scope

• Review docs/BENCHMARK-CONTRACT.md, docs/RESULT-ARTIFACT-SCHEMA.md, and docs/RUN-MANIFEST-SCHEMA.md.
• Add cross-links where one document references concepts defined in another.
• Keep edits limited to navigation and short context notes.

Acceptance criteria

• Relevant schema and contract docs link to each other.
• Added text is concise and does not change the contract meaning.
• Links are relative and point to existing files.

Contributor note

Good first issue for someone comfortable reading structured documentation.